The present invention relates to an additional direct inward dialing connection system and, more particularly, to an additional direct inward dialing connection system using speech to prompt a caller to perform an additional dialing operation when connection to a station terminal cannot be established in a direct inward dialing function from an office line in an automatic private branch exchange.
As a system for implementing an extension telephone in an automatic private branch exchange to terminate a call from an office line, a direct inward dialing connection system is generally used. In this direct inward dialing connection system, when the extension of a telephone is dialed after the user of an originating telephone set connected to an office line dials a direct inward dialing number, a call is made to the extension telephone in an idle state. To remedy the situation where a called extension telephone set is in a busy state, an additional direct inward dialing connection system for accepting additionally dialed extensions corresponding other telephone extensions are available. Conventional direct inward dialing connection systems will be described below.
In a first conventional method, the "PBX Direct Inward Dialing Connection System" disclosed in Japanese Patent Laid-Open No. 3-123295, a PBX exchange capable of performing PBX direct inward dialing connection includes a means for receiving an additionally dialed PBX extension number which is output from an originating telephone set to designate a telephone set other than the telephone set which is in a busy state for which a connection was attempted. This first conventional method also provides a means for connecting the originating telephone set to the extension telephone set for which the additional dialing operation has been performed, in accordance with the received PBX extension number. When a telephone set or telephone set group is in a busy state, the originating telephone set can be connected to another extension telephone set for which an additional dialing operation is performed.
In a second conventional method, the "Office Line Terminating Connection System" disclosed in Japanese Patent Laid-Open No. 60-84090, an automatic private branch exchange also accommodates an office line allowing an additional dialing operation. When office line connecting is attempted with respect to another office line while all input registers for receiving additionally dialed signals are in a busy state, the office line subscriber on the originating telephone set is notified by audible means that the call is being transferred to an attendant console or office line termination display panel. Information about the call is displayed on the attendant console or office line termination display panel and the office line subscriber is precluded from performing re-dialing operations including additional dialing operations.
In the first conventional method, when a telephone set or telephone set group is in a busy state, a call is attempted to another extension telephone set on the basis of the additionally dialed number designated by the caller. If, therefore, this substitute extension telephone is also in a busy state, an additional dialing operation or re-dialing operation must be performed.
The second conventional system is limited to a case wherein while all the input registers for receiving additional dial signals from office line subscribers are in a busy state, an office line subscriber is notified by speech that the call is being transferred to the attendant console or office line termination display panel. However, any description about application of the system to cases wherein a called extension terminal is in a busy state or lock-out state cannot be recognized.